1. Field of the Invention
This invention relates to control devices for reversible D.C. electric motors to protect them against damage upon reversal. More particularly, it concerns such devices that can be used in harsh environments, e.g., submarine systems, require the motor armature to reach a low rotational speed before allowing control circuitry to reverse the motor's rotational direction and protect against over current shutdown for conventional H-bridge motor drives.
2. Description of the Prior Art
Direct current, reversible, permanent magnet motors are often used in harsh environments such as submarine propulsion systems, remote location robotic controllers, etc. They can be subject to instantaneous reversal commands or mechanical stalling which can result in damage or complete destruction of the motor and drive train components. Hence, there is a need to protect the motors in such situations against such damage.
It is known in the art to use the E.M.F. generated by a coasting electric motor to prevent instantaneous reversal using an arrangement of electromechanical relays, e.g., see U.S. Pat. No. 4,365,189.
Use of voltage sensors for detecting counter-E.M.F. induced in the armature, Hall-effect devices in the magnetic field of the stator and speed sensors have been disclosed for use with H-bridge motor configurations in providing D.C. motor control and protection as shown by U.S. Pat. No. 4,338,552.
Electric motor protection on reversal has also been attained with the use of contactors (relays) that can not be operated until the current flowing in the motor is below a predetermined level, see U.S. Pat. No. 3,980,934.
Further, it is know to use opto-couplers, i.e., light emitters connected to photo-detectors via fiber-optic units to control commutation sequence of electric motor controllers (see U.S. Pat. No. 4,170,750) and to turn motors on or off from a remote location (see U.S. Pat. No. 4,359,672).
In spite of the extensive prior innovations in electric motor control devices and the use of opto-couplers in electric motor control, a need exists for further improvements in control devices for reversible D.C. electric motors to protect them against damage upon reversal, particularly for motors intended for operation in harsh environments where the known relevant devices are ineffective or unuseable.